1. The Field of the Invention
The present invention relates generally to orifice fittings using an orifice disk to measure the volume of gas passing through a pipeline. More specifically, the present invention relates to orifice fittings structured for safe removal of the orifice disk during continuous flow of the gas through the pipeline.
2. The Prior Art
Natural gas is generally sold by volumetric measurement. One way to measure the volume of a flowing gas is to pass the gas through a constricted orifice of a known size and measure the pressure drop across the orifice. An orifice fitting is an apparatus used to measure the volume of gas flowing through a pipeline by such means.
A typical orifice fitting comprises a body having a body chamber with a flow passage therethrough in communication with a gas pipeline. Positioned within the body chamber and aligned with the flow passage is an orifice disk having an orifice of a known size through which the gas flows. Pressure taps are fitted immediately upstream and downstream of the orifice for measuring the pressure drop across the orifice disk, thereby permitting calculation of the volume of flowing gas. Because the orifice is subject to wear by impacting of sand, scale, dirt, and other foreign particles in the flowing gas, the orifice disk must be replaced at frequent intervals to insure accuracy in measurement.
In more sophisticated types of orifice fittings, the orifice disk is located in a carrier which can be moved to a service position in the body chamber that is sealed off from the flow passage. The sealed chamber is referred to as the carrier chamber. Once the carrier is moved in the service position, an access opening in the wall of the body can be exposed by removal of a sealed hatch to enable service or replacement of the orifice disk without having to shut off the flow of gas through the flow passage. Such a feature is valuable to pipeline or production facilities where stopping the gas flow would halt transportation. Furthermore, for those who sell gas, the ability to replace the orifice disk without having to shut off the gas flow helps insure continuous supply.
As a result of high pressures in the gas flow pipelines, removal of the hatch to access the orifice disk can often result in serious or even catastrophic consequences if the carrier chamber is not first properly sealed from the gas flow and the gas within the carrier chamber properly vented. To relieve the pressure within the carrier chamber prior to opening the hatch, a valve passageway is provided in the body extending from the exterior of the body to the body chamber. A transfer duct is also positioned within the body and extends from the carrier chamber, when the carrier is in the service position, to the valve passageway. A valve assembly is then positioned in the valve passageway and can be selectively advanced or contracted into a first or second position.
With the valve assembly in the first position and the carrier in service position, gas communication exists between the carrier chamber and body chamber via the transfer duct and one end of the valve passageway. For removal of the orifice disk, the valve assembly is placed in the second position which closes communication between carrier chamber and body chamber but opens gas communication between the transfer duct and opposing end of the valve passageway, thereby permitting the gas within the carrier chamber to travel through the valve assembly and exit the body through the exposed end of the valve assembly.
After the gas within the carrier chamber is bled off, the hatch can be removed and the orifice disk inspected or replaced. Once a disk is replaced and the hatch again secured, the valve assembly can be placed back into the first position which permits repressurization of the carrier chamber. Repressurization of the carrier chamber permits for easy assimilation of the orifice disk back into the gas flow line.
To seal off gas communication with the body chamber as the valve assembly is moved between the first and second position, O-rings are positioned inside the valve assembly. One problem with such rings, however, as shown in U.S. Pat. No. 4,476,730 entitled "Orifice Meter With Pressure Venting and Equalizing Safety Device" issued to Richard S. Brumm on Oct. 16, 1984, is that as the valve assembly is advanced or retracted for opening the respective passages, the O-rings are removed from their adjacent external supports. In turn, the gas flowing through the passages will often dislodge the O-rings from their seat, thereby inhibiting proper seal or advancement of the valve assembly.
An additional problem with the above described valve assembly is that the gas from the carrier chamber is vented through the external end of the valve assembly at the point where the valve is manually advanced or retracted into the desired first or second position. Often the gas within the flow line is poisonous and is usually highly flammable. Accordingly, it is undesirable to have the gas vent into the face of the individual advancing or retracting the valve assembly. Thus, what is needed is an orifice fitting that permits venting of the carrier chamber at a location remote from the valve assembly and, preferably, at a location remote from the orifice fitting itself.
Furthermore, as previously discussed, to safely open the hatch that seals the access opening, the carrier chamber must be properly sealed from the pressurized gas within the body chamber. To seal the carrier chamber, O-rings are positioned to encircle the carrier chamber and to be biased between carrier chamber and the body walls of the body chamber. Over time, however, as the carrier is moved between the service position and the gas flow measuring position, the O-rings can wear, preventing a complete seal. In addition, material in the gas flow line can damage or get lodged against the O-rings, also preventing a proper seal.
Failure of the O-rings to properly seal precludes the orifice chamber from completely venting. If an O-ring fails, high pressure gas is able to pass from the flow passage into the carrier chamber, thereby precluding opening of the hatch. Under such situations, the gas flow line must be completely shut off, thereby defeating the purpose of the valve assembly and access chamber. Accordingly, what is needed is an orifice fitting that includes an alternate, back-up means for selectively sealing the carrier chamber from the body chamber upon failure of the O-rings.